Sacrificial anode bonded with epoxy resin



1965 G. P. CANEVARI 3,202,596

SACRIFICIAL ANODE BONDED WITH EPOXY RESIN Filed Nov. 2.. 1961 Gerard P. Cunevori Inventor Patent Attorney United States Patent 3,2825% SACRIFICIAL ANDDE BGNDED WET EPOXY RESIN Gerard P. Cauevari, Cranford, NJ., assignor to Esso Research and Engineering Company, a corporation of Delaware Filed Nov. 2, 1961, Ser. No. 149,579 6 Claims. ((11. 204-148) This invention relates to a means for preventing electro chemical corrosion of a metallic structure. More specifically, it relates to an improved sacrificial anode and a new method of attaching it to metallic structures to thereby reduce electrochemical corrosion.

When metallic structures remain in contact with a moist ionic environment, as for example ocean-going ships and the like, corrosion of the metallic parts occurs as a result of electrochemical act-ion. This corrosion is minimized by affixing to the structure, blocks of metal of a relatively high electromotive or electrochemical activity. These blocks of metal minimize corrosion of the main structure by disintegrating in preference to the less active metal to which they are attached. Thus they are called sacrificial anodes.

sacrificial anodes are conveniently attached to metallic structures by means of brackets which are bolted or welded in place. It has been found that as these sacrificial anodes corrode, they become loose in their brackets and fall off. Not only does this necessitate the expense of replacing the anodes, but when these anodes are placed in the interior of a structure, they may spark when they strike against a metal surface beneath them. There is much concern regarding this latter effect as it is a possible cause for oil tanker explosions in cargo compartments that are not gas free.

The present invention provides a method of forming a sacrificial anode which eliminates the problems discussed above and which also provides other advantages. In brief, the invention comprises a metal plate with a discontinuous surface, which surface is bonded by means of an epoxy resin adhesive to the substantially continuous surface of the metal structure to be protected. The metal plate, which must be composed of a metal which is higher in the electromotive series than the metal of the structural member, will corrode only on its exposed surface until it is worn away down to the glue line. Thus there is no danger of the plate becoming detached. Furthermore, this type of bonding permits the use of anodes with a high surface/volume ratio since the anode would be bonded to the structural member over its entire surface. Thus more economical anodes of lower activity may be used than is effective with the conventional welding or bolting type of attachment. Further advantages will become apparent during the course of the following discussion.

The metallic structures to which these anodes may "be attached may be made of any suitable metal or alloy, so long as an anode of greater electromotive activity is available. Ordinarily, however, the ferrous metals are preferred and of these, steel is especially preferred. Typical structures to which the invention may apply include heat exchangers, pipelines, and especially ships. The anodes may be applied to either the exterior or interior surfaces of these structures.

The epoxy res-ins to be used as an adhesive in this invention are known in the art to be produced by the reaction of an epihalohydrin, preferably epichlorohydrin, and a dihydric phenol, preferably bisphenol A. The moi ratio of epichlorohydrin to bisphenol A may be varied to produce epoxy resins with different molecular Weights.

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For example-- Epoxy resin, molecular weight Mol ratio, epichlorohydrin bisphenol A:

For this invention, the epoxy resin would preferably be of 45 0+ 500 molecular weight. Molecular weights for epoxies up to 3000, however, are common.

The reaction is catalyzed by the presence of a caustic such as sodium hydroxide, and while some exothermic heat is produced, additional external heat may be applied to bring the reaction temperature to a level of about 175 to 235 C., preferably 215 to 225 C.

The epoxy resins which result from the above reaction have an indefinite shelf life. However, they may be cured into thermoset compounds with the use of curing agents. The usual curing agents are organic polybasic acids, acid anhydrides, polyamides, polyamines, and their adducts. When the epoxy resin is to be used as an adhesive as r in this invention, polyamide resins are preferred. The

polyamide resins which are particularly useful curing agents for this invention include those products which are prepared by the condensation of unsaturated fatty acids and alkyl polyamines. These condensation polymers contain a multiplicity of reactive centers for combination with the epoxide groups of the epoxy resins. About 0.5 to 1 part of polyamide per part of epoxy resin by weight may be used in this invention. The degree of activity of these polyamide resin curing agents is conveniently represented by their amine numberi.e., the number of milligrams of KOI-I equivalent to the base content of 1 gram of polyamide as determined by titration with HCl. Amine values of between and 320 are common. Although any of the above-defined curing agents may be used in this invention, particularly preferred is the polyamide which is the condensation product of dilinole-ic acid and diethy-lene triamine which has an amine number bet-ween about 210 to 230.

Because of the relatively short pot life available with most curing agents, a two-container system is generally utilized in applying epoxy resin adhesives. However, it is possible to utilize a one-container system if curing agents are utilized which do not release amines in the absence of added heat. Dicyandiamine and boron trifiuoride-amine complexes are examples of these latent curing agents.

The epoxy resin adhesives may also be loaded with up to 200 parts by weight per parts of resin with fillers such as mica, talc, short-fiber asbestos, clay; or powdered metallic dust such as zinc or aluminum dust. These fillers are useful in controlling flow, reducing the coefiic-ient of thermal expansion, and reducing the already low rate of shrinkage. For purposes of this invention, fillers of metallic dust will improve the electrical conductivity of the adhesive.

The advantages of epoxy adhesives are known in the art, and those that make them particularly suitable for this invention include the following: They may be cured at room temperature; they are extremely resistant to chemical and salt water corrosion; they may be bonded with minimal contact pressure; the polarity of their groups serves to create strong electromagnetic bonding forces between the epoxy molecule and the metal surface; they shrink to a minimal extent upon setting; shear strengths of about 1500 to 3000 psi. are possible.

The metal which may be used as the anodic plate of this invention must be higher in the electromotive series than the metal of the structure to be protected. When this structure is of a ferrous metal such as steel, the plate is preferably made of magnesium, aluminum or zinc. The bonding surface of this metal must be discontinuous so that points of metal touch the structural member, thereby providing a means for electrical contact. The ridges or anchor pattern on the plate should be from 1 to 20 mils deep, preferably to mils.

This invention maybe better understood by reference to theaccompanying drawings in which:

FIGURE 1 represents a cross-section through a vertically positioned anode, and

FIGURE 2 represents a top view posed anode.

In FIGURE 1, structural member 10, which in this example is a steel bulkhead, has been cleaned before afiixing the plate thereto. Wire brushes, chemical cleanof horizontally disers, power scaling tools, and the like may be used although sand blasting is preferred. Anode 11 maybe between 12 and 60 inches long, 6 and 30 inches wide, and 0.5 to 4 inches thick. In this example it is 24 inches long, 6 inches wide, and 0.5 inch thick. Ridged surface 12 of the anodic plate was also cleaned and an epoxy adhesive was applied thereto. Although many kinds of epoxy adhesives may be used, it is preferred to use a putty:

type epoxy adhesive composed of the'reaction product of epichlorohydrin and bisphenol A inthe mol ratio of about 2/l to which a filler of about 31 wt. percent of clay and approximately 1.5 wt percent of powdered aluminum are added. The resulting filled resin is'then cured with the condensation product of dilinoleic acid and diethylene triamine, as heretofore defined, in an amount of about 70 parts of curing agent by weight per 100 parts of epoxy resin. Ridged surface 12 has an anchor pattern which is'about 10 to 15 mils deep, i.e. the distance from the trough to the crestof each ridge is about 10' to 15 mils. 'After application of the adhesive, the plate is pressed'into place and allowed to set. Mere contact pressure is suflicient although it is preferred to use a clamp. Ridged surface 12 actsto prevent extrusion of the adhesive and thereby maintains acontrolled glue line.

FIGURE 2 shows structural member as being horizontally disposed. Anode or anodic plate 21 is here set with its broad face directly on top, of the structural member. Glue line 22, which extends slightly beyond the limits of anode 21, serves to insulate the interior SUP metallic structural member having a substantially continuous surface, (2). an epoxy resin adhesive produced by the reaction of an epihalohydrin and a dihydric phenol as a bonding medium, (3) a'metallic plate, the metal of which is higher in the electromotive series than the metal of the structural member, said plate having a discontinuous surface containing ridges l to 20 mils deep wherein said adhesive serves to bond the discontinuous surface of the plate to the continuous surface of the structural member and wherein the peaks of said ridges on the metallic plate touch the structural member thereby providing elec trical contact. a 1

2; A claim according to claim 1 wherein the metal of the structural member is steel and the metallic plate attached thereto is formed from a metal selected from the class consistingof magnesium, aluminum, and zinc.

3. A claim according to claim 1 wherein the epoxy resin adhesive is composed of the epoxy resin which is the reaction product of epichlorohydrin and bisphenol A in the mol ratio of about 2/1 to which is added a filler of about 31 wt. percent of clay and about 1.5 wt. percent of aluminum powder, said epoxy resin being cured by about 70 parts by weight of curing agent per 100 parts of epoxy resin, said curing agent being the reaction product of dilinoleic acid and diethylene triamine which has an amine number between'about 210 and 230.

4. A process for attaching to a metallic structure a plate of a metal of higher electromotive activity comprising: (1) cleaning a portion of a relatively continuous 7 the structure.

face of the anodic plate from corrosion and thus insures that the plate will not become detached. I

The advantages of this invention include the following: The anodes as herein described may be installed more cheaply since, for example, it is not necessary to free bulkheads of gas in order to weld. V

I Since the anode shape makes possiblea high surface/ volume ratio, less active and cheaper materials such as zinc and aluminum may be used.

.The problem of falling anodes is eliminated, thereby reducing the-oost of replacement and eliminating the hazard of sparking. I 4

, This invention has been described in connection with certain specific embodiments thereof; however, it should be understood thatthese are by way of example rather than by way oflimitation, and it is not intended that the invention be restricted thereby, but only by the scope of the appended claims.

, What is claimed is:'

1.v A corrosion inhibited structure comprising: '(1) a;

5. A process according to claim 4 wherein the metal of the structure is steel and the. metallic plate attached thereto is formedfrom a metal selected fromthe class consisting of magnesium, aluminum and zinc.

6. 'A process accordingto claim 4 wherein the epoxy resin adhesive is composed of the epoxy resin which is the reaction product of epichlorohydrin and bisphenol A in the' mol ratio of about 2/1 to which is added a filler of about 31 wt. percentof clay and about 1.5 wt. percent of aluminum powder, said epoxy resin being cured by about 70 parts by weight of curing agent per 100 parts of epoxy resin, said curing agent being the reaction prodnot of dilinoleic acid anddiethylene triamine which has an amine number between about 210 and about 230.

References Cited by the Examiner UNITED STATES PATENTS WINSTON A. DOUGLASQPrimary Examiner.

EARL M. BERGERT, MURRAY TILLMAN,

- Examiners. 

4. A PROCESS FOR ATTACHING TO A METALLIC STRUCTURE A PLATE OF A METAL OF HIGHER ELECTROMOTIVE ACTIVITY COMPRISING: (1) CLEANING A PORTION OF A RELATIVELY CONTINUOUS SURFACE OF THE STRUCTURE, (2) COATING ONE SURFACE OF THE METALLIC PLATE WITH AN EPOXY RESIN ADHESIVE PRODUCED BY THE REACTION OF AN EPIHALOHYDRIN AND A DIHYDRIC PHENOL, SAID SURFACE CONTAINING A PLURALITY OF RIDGES, 1 TO 20 MILS DEEP, AND (3) PRESSING SAID COATED SURFACE TO THE CLEANED CONTINUOUS SURFACE OF THE METALLIC STRUCTURE SO 